The Third Generation Partnership Project (3GPP) has developed standards for a cellular communications system, referred to as Long Term Evolution (LTE). LTE is based on transmission using Orthogonal Frequency Division Multiplexed (OFDM) signals.
An OFDM signal comprises a plurality of subcarriers, each of which is modulated to carry a transmission symbol. Data for transmission is divided into a plurality of parallel streams, one for each subcarrier. The transmission symbol for each subcarrier is generated by modulating the data using a conventional modulation scheme. Advantageously, OFDM simplifies channel equalisation, by adapting the modulation scheme applied to each subcarrier according to the narrowband propagation conditions for that subcarrier.
In cellular systems, such as LTE, a mobile station estimates the downlink radio channel in order to coherently demodulate information transmitted to it from the base station. In LTE, the base station inserts reference symbols (RS) into the OFDM signal that it transmits. These are used for channel estimation by the mobile stations. At least one cell-specific reference symbol is transmitted by a base station in a given time period.
3GPP standard TS 36.211 Release 9 defines the transmission of reference symbols in the downlink. The reference symbols are transmitted over the full downlink channel bandwidth for the cell covered by the base station. Mobile stations measure the reference symbols transmitted by the base station providing them with service. They additionally measure the reference symbols transmitted by other base stations. This is useful for handover purposes, when moving between cells covered by different base stations.
It is desirable that each mobile station receives the cell-specific reference symbols from each relevant base station, in order to ensure that it can properly receive data. Consequently, power allocation and interference management for the transmission of reference symbols is considered by existing standards. In particular, it is desirable to avoid reference symbols transmitted by two different base stations overlapping in both time and frequency. This is termed a reference symbol collision.
Since reference symbols are repeatedly transmitted by the base stations, independently from user data, reference symbol collision leads to constant interference. Any attempt to boost the power allocated to the base stations reference symbols will result in increased interference to other base stations reference symbols.
3GPP standard TS 36.211 Release 9 provides for frequency shifting of one base station reference symbol to avoid overlap. In other words, the subcarriers that are used to carry the reference symbols can be changed slightly. Nevertheless, only a limited number of frequency shifts are possible using such a technique. Moreover, such a technique requires advanced planning of resource allocation to reference between different base stations. Avoiding reference symbol collisions without these complications is a particular difficulty.